An optical disk such as a digital audio disk, i.e., a compact disk, or a video disk is fabricated by forming an aluminum reflective film on a transparent substrate and forming a protective film or the like on the reflective film. Optically readable information is stored in the substrate in the form of phase pits.
In such an optical disk, a reading light, typically in the form of a laser beam, is directed onto the surface of the optical disk to make it possible to detect a great reduction in the amount of reflected light at the pits due to diffraction of the light. Thus, information can be read from the disk, i.e., the original signal is reproduced.
In the above-described optical disk, the resolution at which the signal is reproduced is determined almost only by the wavelength .lambda. of the light emitted from the light source of the reading optical system and by the numerical aperture NA of the objective lens. When the period of the produced phase pits exceeds the diffraction limit .lambda./2NA, the original signal is reproduced well.
Therefore, in order to store information in such an optical disk at a higher density, it is necessary for the reading optical system to use a semiconductor laser emitting a shorter wavelength .lambda. of light as its light source and to increase the numerical aperture NA of the objective lens.
However, limitations are imposed on the shortening of the wavelength of the light source and on the increase in the numerical aperture of the objective lens. In particular, if the wavelength of the light source is shortened, the recording density can be increased by a factor of four at best at the present level of technique. If an attempt is made to increase the numerical aperture of the lens, then it is difficult to manufacture lenses having a small amount of aberration. Even if such a lens is obtained, the stability of the focus against vibration and skew of the disk deteriorates. Consequently, the present situation is that it is difficult to improve the recording density of optical disks drastically.
We have already proposed optical disks capable of offering resolutions exceeding the aforementioned limit imposed by the wavelength .lambda. of the light source or by the numerical aperture of the lens system in Japanese Patent applications Ser. Nos. 94452/1990 and 291773/1990. More specifically, the phase within the laser spot of the reading light is partially varied to change the reflectivity, thus permitting reproduction at an ultrahigh resolution.